Electric machines for power generation such as used in wind turbines for instance comprise a stator and a rotor, the latter being rotatable relative to the stator. During the operation of the electric machine, that is a generator for instance, magnetic fields originating from permanent magnets or wound poles attached to the rotor induce electrical currents into the stator cores and coils in order to generate electrical power. Thereby, large amounts of excessive heat occur due to the induction of current within the stator.
Therefore, it is known to provide the stator or a stator element respectively with cooling means providing cooling of the stator by means of air ventilation, the provision of fins projecting radially from the stator giving rise to an enlarged surface of heat exchange or liquid cooling, whereby a liquid cooling medium like water for instance is conveyed through separate cooling pipes engaging with the stator. The latter principle is described in US 2007/0024132 relating to a method and an apparatus for cooling wind turbine generators.
Aside, additional excess heat is produced by eddy currents usually being generated when a conductor is moved within an inhomogeneous magnetic field or alternatively due to time-dependent variations of a magnetic field. Eddy currents generate magnetic fields opposing the original magnetic field and thus, resulting in a so called eddy current loss. In large scale wind turbines, eddy current losses may reach a significant level, the efficiency of the generator is usually decreased.
A method for minimizing said eddy currents in a wind turbine stator is disclosed in JP 2001-54271 A referring to a brushless motor having a yoke being split in blocks, whereby sheets of insulating paper are disposed between the blocks.
Yet, these methods do not provide sufficient cooling of the stator of an electrical machine like a generator for instance, particularly in times of high-performance.